The present invention relates to a method of detecting diaphragm excursion or displacement of electrodynamic loudspeakers and a corresponding loudspeaker excursion detector. Methodologies and devices for detecting diaphragm excursion of electrodynamic loudspeakers are highly useful for numerous purposes for example in connection with diaphragm excursion control or limitation. Diaphragm excursion control is useful to prevent the diaphragm and voice coil assembly being driven beyond its maximum allowable peak excursion. Unless proper precautionary measures are taken, powerful amplifiers may force such high levels of drive currents into the voice coil that the diaphragm and voice coil assembly is driven beyond its maximum allowable peak excursion leading to various kinds of mechanical damage. Hence, there is a need to monitor/detect the instantaneous displacement of a loudspeaker diaphragm to prevent mechanical damage caused by excursions exceeding the excursion limit of the type of electrodynamic loudspeaker in question. This issue is of significant importance in numerous areas of loudspeaker technology such as high power loudspeakers for public address systems, automotive speaker and home Hi-Fi applications as well as miniature loudspeakers for portable communication devices such as smartphones, laptop computers etc.
Many attempts have been made in the prior art to detect or estimate instantaneous displacement of loudspeaker diaphragms for the above outlined purposes. These attempts have often been based on complex non-linear models of the particular loudspeaker type in question. Model-based approaches require careful analysis of the electro-mechanical and magnetic characteristics of the particular loudspeaker type of interest. Likewise, model based approaches require complex real-time computations on the non-linear loudspeaker model to estimate the actual excursion of the real operative loudspeaker. Complex computations leads to high power consumption of a Digital Signal Processor executing the model based estimate and/or control algorithm which is particularly undesired for battery powered communication devices like smartphones etc. The model parameters can furthermore be difficult to determine accurately and may vary over temperature, time and between individual loudspeaker samples of the same type. Other attempts have been based on transducer signals supplied by various types of acceleration and velocity sensors attached to the diaphragm or voice coil.
Hence, it is of significant interest and value to provide a relatively simple method for estimating or detecting the displacement or excursion of the loudspeaker diaphragm without relying on complex non-linear models of the particular loudspeaker type. The displacement detection may be accompanied by a suitable mechanism for limiting the diaphragm displacement if it exceeds the loudspeaker's maximum allowable peak excursion. The diaphragm excursion detection mechanism and the corresponding detector should preferably be operative with minimal, or without, a priori knowledge of linear and non-linear properties of the loudspeaker to simplify or entirely eliminate calibration procedures.
EP 2 453 670 A1 discloses a method to generate a control signal that can be used for mechanical loudspeaker protection or for other signal pre-processing functions in a loudspeaker control system without requiring knowledge of the mechanical parameters of the loudspeaker. The control signal may be a measure of how close the loudspeaker is driven to its mechanical displacement limit and is based on a so-called arbitrarily scaled frequency dependent input voltage to excursion transfer function. The latter transfer function is derived during a calibration procedure from a plurality of drive voltage and current measurements on the loudspeaker at different audio frequencies.
U.S. 2009/268918 A1 discloses mechanical protection of loudspeakers using digital processing and predictive estimation of instantaneous displacement of the voice coil in a loudspeaker transducer. The invention solves the problem of limiting the voice coil displacement of the transducer by applying a look-a-head based linear or non-linear predictor and a controller operating directly on the displacement signal in order to finally convert back into the incoming signal domain.
U.S. Pat. No. 5,931,221 B1 discloses with reference to FIG. 7, a dynamic loudspeaker driving apparatus which comprises a power amplifier coupled to an electrodynamic loudspeaker and a feedback circuit for providing improved motional feedback. The feedback circuit negatively feedbacks the detected motional voltage to the power amplifier. A bridge circuit is used to extract a motional voltage produced by the loudspeaker. A leg of the bridge includes an impedance which corresponds to the impedance of the dynamic loudspeaker including its motional impedance so to provide a more accurate motional feedback voltage.